Check for unused attribute.

m4/alberta-unused.m4

+# Check if __attribute__((unused)) is supported
+# Stolen from dune-common (thanks)
+
+AC_DEFUN([ALBERTA_CHECK_UNUSED],[
+    AC_CACHE_CHECK([for __attribute__((unused))], dune_cv_attribute_unused, [
+        AC_LANG_PUSH([C++])
+        AC_COMPILE_IFELSE([AC_LANG_PROGRAM([[
+                        #define UNUSED __attribute__((unused))
+                        void f(int a UNUSED, int UNUSED)
+                        {
+                          int UNUSED b;
+                        }
+                          ]],
+                          [])],
+                        dune_cv_attribute_unused="yes",
+                        dune_cv_attribute_unused="no")
+        AC_LANG_POP([C++])
+    ])
+
+    AS_IF([test "x$dune_cv_attribute_unused" = "xyes"],
+      [AC_DEFINE_UNQUOTED(HAS_ATTRIBUTE_UNUSED,
+                          1,
+                          [does the compiler support __attribute__((unused))?])],)
+])